Human bronchus, pancreatic duct and esophagus can be cultured for several weeks in a chemically defined medium. This controlled experimental setting provides an excellent in vitro system to study the metabolism of chemical carcinogens, including those found in tobacco smoke and the environment. Several classes of chemical carcinogens, polynuclear aromatic hydrocarbons, N-nitrosamines, hydrazines, aromatic amines and mycotoxins can be metabolically activated by human tissues. Epithelial cell cultures initiated from human bronchus and esophagus metabolized benzo(a)pyrene (BP), aflatoxin B1 (AFB) and N-nitrosodimethylamine (DMN) into species which reacted with cellular DNA. The metabolic pathways leading to the formation of DNA adducts in explant and epithelial cell cultures have been defined by BP, 7,12-dimethylbenz(a)anthracene, AFB, and DMN. The pathways to organic-extractable and water-soluble metabolites of BP were qualitatively similar in all 3 tissues. Th adducts between these carcinogens and DNA in human bronchus andesophagus are essentially the same as those found in experimental animals in which the chemicals are carcinogenic. Interindividual differences in carcinogen-DNA binding values vary 50- to 150-fold.